1. Field of the Invention
The present invention relates to a so-called biological filtration apparatus which simultaneously carries out decomposition of organic impurities by microorganisms and filtration thereof while using primary treatment water or secondary treatment water as raw water (water to be processed) in water purification/refinement processing.
2. Related Background Art
There have conventionally been known biological filtration apparatus using a biological tank filter comprising a microorganism-attached particulate carrier as a fixed bed (packed bed), in which raw water for secondary treatment having completed primary purification processing is introduced into the biological tank filter from the upper part thereof and then moves downward due to gravity, whereas air is diffused from the lower part of the biological tank filter at the same time, the water being purified under the action of aerobic microorganisms as the air moves to the upper part in the countercurrent direction of the movement of the raw water for secondary treatment, and solids are filtered out so as to discharge only the purified liquid. Also, water or air has been supplied from the lower part of the biological tank filter upon an increase in pressure loss in the fixed bed and the like, so as to effect backwashing (e.g., Japanese Patent Publication No. SHO 57-37399).
Such conventional biological filtration apparatus have employed carbonic or inorganic materials having a true specific gravity of about 1.1 to 1.6 such as active carbon (true specific gravity: 1.15 to 1.28), anthracite (true specific gravity: 1.4 to 1.6), and burned clay (chamotte) (true specific gravity: 1.1 to 1.5) as a particulate carrier.
In the case of the above-mentioned biological filtration apparatus for processing the raw water for secondary treatment, however, the particulate carrier packed in the biological tank filter comprises solid particles having a diameter of about 2 to 6 mm, whereby the watering space formed between the carrier particles in the packed bed is so small that the capacity of purification processing may not fully be exerted. Also, the ventilation space for the air diffused for growing the microorganisms attached to the particulate carrier is so small that pressure loss is large.
In order to secure their required watering space, on the other hand, the above-mentioned conventional biological filtration apparatus using the particulate carrier have periodically carried out cleaning of the particulate carrier, so-called backwashing, by causing cleaning air (backwashing air) or cleaning water (backwashing water) to flow in the direction opposite to the usual flow of the water to be processed, since the watering space further decreases as their operation continues, due to the microorganisms, decomposition products, and the like adhering to the particulate carrier. However, the above-mentioned conventional biological filtration apparatus have been problematic in that they necessitate a large amount of backwashing water (usually 40 to 60 m3/m2xc2x7h) and a large amount of backwashing air (usually 50 to 60 m3/m2xc2x7h).
It is an object of the present invention to provide a biological filtration apparatus which is:
(1) capable of enhancing the watering space and ventilation space in the packed bed filled with carrier particles for biological purification processing so as to lower the pressure loss in the biological filtration apparatus, while reducing the amount of backwashing water and backwashing air and the backwashing pump power at the time of backwashing, thereby being able to decrease the number of backwashing operations; and
(2) capable of improving the adherence of microorganisms to the surface of carrier particles for biological purification processing, so as to improve the quality of the processed water.
The biological filtration apparatus in accordance with the present invention comprises:
a raw water introducing section for introducing raw water as water to be processed;
a biological filtration section, disposed on the lower side of the raw water introducing section, for biologically purifying/refining/filtrating the raw water so as to yield processed water;
a support section, disposed on the lower side of the biological filtration section, for supporting the biological filtration section, the support section having a liquid and gas permeability; and
a water collecting section, disposed on the lower side of the support section, for collecting the processed water;
the biological filtration section including a packed bed comprising a hollow carrier particle, made of a resin, having a true specific gravity of at least 1.01 g/ml but less than 1.2 g/ml, preferably at least 1.01 g/ml but less than 1.1 g/ml.
The hollow carrier particle employed in the biological filtration apparatus of the present invention is not limited to one made of a resin (plastic material) alone, but is preferably formed by a mixture of a resin and an additive for adjusting specific gravity selected from the group consisting of inorganic and organic substances. As such an additive for adjusting specific gravity, one selected from the group consisting of pulp, waste plastics, zeolite, barium sulfate, and slaked lime is preferably used.
As the hollow carrier particle employed in the biological filtration apparatus of the present invention, a tubular carrier particle having an outside diameter of 0.5 to 10 mm, an outside diameter to length ratio (aspect ratio) of 1/0.5 to 1/3, and a thickness of 0.1 to 3 mm is preferably used.
By employing the above-mentioned configuration, the biological filtration apparatus of the present invention yields the following operations and effects:
1) Since the carrier particle used in the biological filtration apparatus of the present invention is made of a resin, a hollow carrier particle having a desirable shape and a true specific gravity within the above-mentioned range is obtained easily. Since this carrier particle is hollow, the packed bed filled with such carrier particles has a higher void ratio and a lower packing density as compared with the packed bed filled with solid carrier particles, whereby the apparatus as a whole has a lighter weight.
2) Since the hollow carrier particle has a larger specific surface area, a larger amount of microorganisms attaches thereto, whereby biological purification and filtration capacities would improve.
3) The microorganisms attached to the hollow inside of the hollow carrier particle are stable, and are washed out very little at the time of backwashing, whereby a stable organism holding capacity is achieved.
4) Since the packed bed filled with the hollow carrier particles has a high void ratio, and the watering space and ventilation space are large, the pressure loss at the time of watering is low. For example, in the case where a cylindrical carrier particle having an outside diameter of 4 mm, an inside diameter of 3 mm, a length of 4 mm, and a true specific gravity of at least 1.01 g/ml but less than 1.2 g/ml is used, under a watering condition substantially the same as that in the case where a solid spherical carrier particle having a particle diameter of 3 mm is used, the pressure loss at the time of watering is small, i.e., xc2xd to ⅕, and the pressure loss at the time of ventilation is similarly small.
5) Since the carrier particle employed in the biological filtration section of the biological filtration apparatus of the present invention has a true specific gravity relatively lower than that of the carrier particle of the conventional biological filtration apparatus and is hollow, it can sediment at the time of biological processing and can flow to the raw water introducing section with a small amount of water or air at the time of backwashing. As a consequence, the power for supplying, under pressure, the air for backwashing and/or diffusing and the water for backwashing can be lowered.
Namely, the hollow carrier particle in accordance with the present invention can reduce the apparent specific gravity per volume (weight per apparent volume). Therefore, backwashing is possible with a small amount of backwashing water, i.e., 5 to 25 m3/m2xc2x7h, whereby an amount of backwashing water which is about ⅕ to xc2xd of the conventional amount of backwashing water is sufficient. Also, backwashing is possible with a small amount of backwashing air, i.e., 15 to 30 m3/m2xc2x7h, whereby an amount of backwashing air which is about xc2xc to xc2xd of the conventional amount of backwashing air is sufficient.
Since the hollow carrier particle employed in the present invention is made of a resin and has a true specific gravity (at least 1.01 g/ml but less than 1.2 g/ml) which is relatively close to that of water, its apparent specific gravity in water is further closer to water. Therefore, the amount of backwashing water or backwashing air can be reduced at the time of backwashing the biological filtration apparatus, and the pressure for backwashing can be lowered, whereby the backwashing pump power can be reduced, and the number of backwashing operations can be decreased.
6) Since the void ratio in the packed bed of hollow carrier particles in the biological filtration section is high, the real space is so large that the water to be processed (raw water) and the hollow carrier particles come into contact with each other for a longer lime, whereby the biological purification and filtration capacities would improve.
The support section in the biological filtration apparatus of the present invention preferably comprises a planar member and a water collecting nozzle arranged in the planar member, whereas such a water collecting nozzle preferably has a configuration as follows. Namely, the upper part of the water collecting nozzle is exposed into the biological filtration section and is formed with a slit having a liquid and gas permeability. The lower part of the water collecting nozzle is exposed into the water collecting section, and is formed with a water outlet/inlet for discharging the processed water and receiving backwashing water and an air inlet for receiving diffusing and/or backwashing air, whereas the water outlet/inlet and the air inlet communicate with the slit. When the support section thus provided with a water collecting nozzle is used, the processed water tends to be efficiently isolated from the biological filtration section and collected by the water collecting section, and the diffusing air tends to be uniformly supplied to the biological filtration section. At the time of backwashing, on the other hand, there is a tendency of backwashing water and/or air to be supplied efficiently and uniformly to the biological filtration section.
The support section in the biological filtration apparatus of the present invention may comprise a first support portion, comprising a packed bed of filter particles having an average particle size of 0.5 to 2.0 mm, for supporting the biological filtration section; and a second support section, comprising a planar member and a water collecting nozzle arranged in the planar member, for supporting the first support portion. Such a water collecting nozzle preferably has a configuration as follows. Namely, the upper part of the water collecting nozzle is exposed into the first support portion and is formed with a slit having a liquid and gas permeability. The lower part of the water collecting nozzle is exposed into the water collecting section, and is formed with a water outlet/inlet for discharging the processed water and receiving backwashing water and an air inlet for receiving diffusing and/or backwashing air, whereas the water outlet/inlet and the air inlet communicate with the slit. When the support section thus provided with a packed bed of filter particles is used, the solid content tends to be eliminated by this packed bed more reliably.
Further, the support section in the biological filtration apparatus of the present invention may comprise a first support portion, comprising a packed bed of filter particles having an average particle size of 0.5 to 2.0 mm, for supporting the biological filtration section; a second support portion, comprising a packed bed of larger particles having an average particle size greater than that of the filter particles, for supporting the first support portion; and a third support portion, having a liquid and gas permeability, for supporting the second support portion. In this case, the water collecting section is preferably made of a block structure connected to the third support portion and partitioned into a plurality of cells. Preferable as such a third support portion is one comprising a planar member, a water discharge/supply tube for discharging the processed water and supplying backwashing water, and an air supply tube for supplying diffusing and/or backwashing air, whereas the water discharge/supply tube and the air supply tube are arranged in the planar member.
Furthermore, the biological filtration apparatus of the present invention may be configured such that the support section comprises a first support portion, comprising a packed bed of filter particles having an average particle size of 0.5 to 2.0 mm, for supporting the biological filtration section; whereas the water collecting section comprises:
a second support portion, comprising a packed bed of larger particles having an average particle size greater than that of the filter particles, for supporting the first support portion;
a first pipe, embedded in the second support portion, for collecting the processed water and introducing backwashing water; and
a second pipe, embedded in the second support portion, for introducing diffusing and/or backwashing air.
As the filter particle in accordance with the present invention, at least one kind of material selected from the group consisting of filter sand, anthracite, and garnet is preferable. When such a filter particle is used, the filtering capacity tends to improve further.
Though the portion where the diffusing air, backwashing air, or backwashing water is introduced is preferably the water collecting portion as mentioned above, it may be a biological filtration section (preferably in the vicinity of the support section).